1. Field of the Invention
This invention relates to an optical element and an optical system having the optical element, and is particularly suitable for the optical system of a video camera, a still camera, a copying apparatus or the like, designed to form an object image on a predetermined surface by the use of an optical element (off-axial optical element) including a curved surface (off-axial curved surface) which is not a flat surface, in which a normal plane does not coincide with a reference axis at a point at where an optical path (reference axis) of a reference wavelength leading from an object plane to an image plane intersects the curved surface of the element.
2. Related Background Art
A coaxial optical system in which a rotation-symmetrical refracting surface or reflecting surface is disposed about an optical axis, which is the rotation-symmetry axis of each surface (a refracting surface or a reflecting surface), has heretofore been used chiefly as an optical system for forming the image of an object on an image plane. Recently, however, an optical system of a new concept, i.e., an off-axial optical system which does not fall within the category of the conventional coaxial optical system and which includes a curved surface (off-axial curved surface) which is not a flat surface, in which a normal plane does not coincide with a reference axis at a point at where an optical path (reference axis) of a reference wavelength leading from an object plane to an image plane is introduced, for example, in pages 49 to 56 of the 20th Optics Symposium Lectures.
Such an off-axial optical system, in contrast with the conventional coaxial optical system, can be defined in a form wherein the reference axis (corresponding to the optical axis of the conventional coaxial optical system) is bent, and the conventional coaxial optical system can be considered to be a special case of this off-axial optical system. Therefore, the degree of freedom of the disposition of the optical system can be increased and besides, the variations of the optical system are expected to become more than those of the coaxial optical system.
On the other hand, as an example in which a diffraction optical element utilizing a diffraction phenomenon is applied to an eccentric optical system (generally, being eccentric refers to a case where the rotation-symmetry axis of a surface which is the xe2x80x9ccorexe2x80x9d is parallel-shifted or is tilted about a certain point, and the eccentric optical system is generally a special example of an off-axial optical system in which an expression representing a surface is constituted by a surface having no rotation-symmetry axis including even the area outside an effective portion), there is an example in which a reflection type hologram is attached to the combiner of the head-up display of an aircraft or an automobile shown in U.S. Pat. Nos. 4,669,810, 4,763,990, 4,799,765, etc.
For such an optical system (off-axial optical system), however, it becomes necessary to take rotation-asymmetrical aberrations which did not need to be considered in the coaxial optical system due to its rotation-symmetry. Therefore, it is necessary that an expression expressing a surface shape be an expression expressing a rotation-asymmetrical shape and the surface shape becomes incapable of being formed by a simple shape such as a rotation-symmetrical surface.
Also, by making the expression of the surface shape into a rotation-symmetrical one, the degree of freedom of the design as the coefficient of the surface shape per surface is increased. However, the number of asymmetrical aberrations to be corrected is also increased and therefore, the total number of refracting surfaces and reflecting surfaces required for aberration correction need be about equal to that in the coaxial system.
Also, in the example introduced in the above cited reference wherein a reflection-type hologram is attached to the combiner of the head-up display of an aircraft or an automobile, all the optical systems are observation optical systems and the number of surfaces used is limited to about two. Therefore, the optical performance for chromatic aberration or the like has not always been satisfactory.
Also, in the above-described example wherein a reflection-type hologram is attached to the combiner of the head-up display of an automobile, the surface (element) having the diffracting action is virtually singly installed on a space, and to obtain high optical performance, it is necessary to exactly effect optical adjustment, such as the alignment, with other optical element.
The present invention has as its object the provision of an optical element and an optical system which are compact and can easily obtain high optical performance.
The present invention is (1-1) an optical element for reflecting light from an incidence surface by one or more reflecting surfaces comprising off-axial curved surfaces and causing the light to emerge from an emergence surface or an optical system having this optical element, in which at least one of the incidence surface, the emergence surface and the reflecting surface diffracts the light (by a diffraction grating thereon).
The present invention is (1-2) an optical element in which a refracting surface of the optical element (an incidence surface) on which a light is incident, one or more reflecting surfaces having off-axial curved surfaces for reflecting the incident light beam, and a refracting surface of the optical element (an emergence surface) from which the light beam reflected by the one or more reflecting surfaces emerges, are formed integrally with one another or an optical system having this optical element, wherein at least one of the incidence surface, the emergence surface, and the reflecting surface diffracts the light (by a diffraction grating thereon).
Particularly, in the construction (1-1) or (1-2), there are aspects characterized in that
(1-2-1) the surface having the diffracting action is a curved surface,
(1-2-2) the surface having the diffracting action is a spherical surface,
(1-2-3) the surface having the diffracting action is a rotation-symmetrical aspherical surface,
(1-2-4) the surface having the diffracting action is a rotation-asymmetrical aspherical surface,
(1-2-5) the surface having the diffracting action is a hologram type diffracting surface,
(1-2-6) the surface having the diffracting action is a kinoform type diffracting surface,
(1-2-7) the surface having the diffracting action is a binary type diffracting surface,
(1-2-8) the surface having the diffracting action is a diffracting surface having a rotation-asymmetrical striped shape,
(1-2-9) the optical path is divided into a plurality of paths by the order of the diffracting action of the surface having the diffracting action, and
(1-2-10) it has a plurality of focal lengths differing depending on the order of the diffracting action of the surface having the diffracting action.
A certain optical system of the present invention is
(2-1) an optical system constituted by a plurality of optical elements and provided with at least one optical element of the construction (1-1) or (1-2).
Particularly, there are aspects characterized in that
(2-1-1) the optical element is moved, whereby the focal length of the total system is changed,
(2-1-2) the optical element is moved along a reference axis,
(2-1-3) it is provided with a coaxial optical element,
(2-1-4) the coaxial optical element is moved, whereby the focal length of the entire system is changed, and
(2-1-5) the coaxial optical element is moved along the reference axis. A certain optical element of the present invention is (3-1) an optical element for causing a light beam to enter an incidence surface, reflecting the light beam from the incidence surface by one or more reflecting surfaces, and causing the light beam to emerge from an emergence surface and in which at least one of the incidence surface, the emergence surface, and the one or more reflecting surfaces is a surface having the diffracting action and formed with rotation-asymmetrical stripes.
A certain optical system of the present invention includes
(4-1) at least one optical element of the construction
(3-1) and forms an object image on a predetermined surface through the optical element.